Why is my BezierCurve shaded like a tube?
Because by default is spline set under
Properties Editor &gt; Data Properties &gt; Active Spline as Smooth
When Smooth unchecked, curve become shaded Flat, which means flat also in curvature.

To make it smoother you can increase Resolution U (segment subdivisions)

But in some cases it would be still segmented ... so Edge Split  modifier seems to be better choise anyway :)